Field telephone system



y 1945- F. LEIBE 7 2,376,367

FIELD TELEPHONE SYSTEM Filed May 13, 1943 2 Sheets-Sh'eet l PECE/V/NGAMPLIFIER May 22, 1945. F. A. LElBE FIELD TELEPHONE SYSTEM Filed May 13,1943 2 SheetsSheet 2 INVENTOR. FAHNK a. 45/55 Patented May 22, 1945 mamrcmrnonc srs'rmu Frank A. Leibe, Qua kertown, Pa., assignor to FederalTelephone and Radio Corporation, New York, N. Y., a corporation ofDelaware Application May 13, 1943, Serial No. 486,755

6 Claims.

This invention relates to telephone systems of the type in which areceiver and a transmitter at one end of a telephone line arealternately switched into operation, an arrangement especially adaptedfor use in field telephones. In systems of this type it is desirablethat an operator who has finished transmitting and is about to switchhis set from transmitting to receiving position shall notify the otheroperator of that fact so that the latter will know when to switch in histransmitter and commence talking. The invention is particularly intendedfor systems in which vacuum tube amplifiers are used at each end of theline.

The general object of this invention is to provide a novel switch-oversignal that will be automatically transmitted from a set at one end ofthe line when the set is switched from transmitting'to receivingposition. A feature of the inventic-n is the utilization of the vacuumtube amplifier circuits for this purpose.

An important object is the provision of an arrangement that willgeneratea switch-over signal without the use of separate signal-producingdevices. This is in general accomplished by creating signal-producingoscillations in the tube circuits for a brief period at the time whenthe set is switched from sending to receiving position. A feature of theinvention is the attainment of this object by the use of simple circuitarrangements automatically producing a brief period of oscillationwhenever the set is switched over to receiving position, and which areadapted for incorporation in highly compact field sets withoutincreasing the size of such sets.

Other objects and advantageswill appear from the following descriptionconsidered in connec tion with the accompanying drawings, in which Fig.1 is a simplified diagram illustrating one form of the invention, and

Fig. 2 is a similar diagram illustrating another embodiment.

Referring to Fig. 1, a handset I is indicated,

including a receiver ll connected by leads l2,

tem which includes a receiving amplifier tube 20 and a transmittingamplifier tube 2|, together with a suitable arrangement for placing inoperation tube 20 when switch I8 is open, and tube 2! when said switchis closed. In theform illustrated in Fig. 1, this is accomplished byalternately energizing the filaments of the tubes, an arrangement thatis advantageous for battery operated field sets since it requires theheating of only one tube filament at a time. This switch arrangement isutilized to produce the switch-over signal, and takes advanta e of thefact that if one of the filaments is disconnected when the otherfilament is simultaneously energized, the former filament will retainits electron-emitting temperature long enough to produce simultaneousoperation of the tubes for a brief period lasting until the temperatureof the cathode of the de-energized tube drops below the temperature ofuseful emission. By providing proper coupling between the circuits ofthe two tubes during this brief period of simultaneous operation,oscillations are produced which are transmitted over the line I! to theother set.

In the form shown, the filament 22 of tube 20 is connected through line23 and filament battery 24a to line H, and its energizing circuit iscompleted through line 25 and upper contact 26 of one pole of switch l8which is connected to line H, said switch being of the double-pole,double-throw type. Operating grid 21 is coupled to transmission line l9through winding 28 of transformer 29, while the anode 30 is connected towinding 3| of transformer 32 in inductive relation to winding 33connected across the induction coil l4 and condenser IS in the handsetcircuits. Tube 20 is preferably provided with a screen grid 34 connectedto an appropriate source of positive potential in the usual way. Withthis arrangement signals coming in on line i9 are impressed throughtransformer 23 on tube 20, amplified, and transmitted throughtransformer 32 to the receiver II.

The filament 36 of transmitter tube 2i is connected at one side throughline 3'! and 23 to handset line I1, and at the other side through line38 to filament battery 24!: and the lower contact 39 of the upper poleof switch l8. Operating grid 40 is connected to winding ll oftransformer 32, while anode 42 is connected to winding 43 of transformer2!]. Tube 2| is advantageously provided with a screen grid 44 connectedto a suitable source of positive potential. With this arrangement thewaves from the circuit of microphone l6 will be impressed throughtransformer 22 on tube 2|, amplified therein and transmitted throughtransformer 22 to line It. The opera- .tion of'push switch II will thusserve not only to shift the handset circuits between transmitting areturn from transformer winding" being ads iustably connected toresistance 48 to provide variable grid bias from filament battery 24b,functioning as a gain control, with the usual condenser in shunt. Theouter end of winding II is connected to a suitable grounded source ofpositive anode potential. The negative feedback circuit for tube 2|comprises the connection of anode 42 through condenser 5i and.resistance 52 to winding ll of transformer 32, the other end of thewinding going to grid 40, the lower end of resistance 52 being connectedthrough parallel resistances 53 and condenser 54 to line 25 and thenegative side of battery 24a which provides bias potential for grid 40.The outer end of winding 42 is likewise connected to an appropriatesource of positive potential. The negative sides of filament battery 24and the sources of positive potential'are of course connected in theusual manner, as by grounding to the metal frame.

With this arrangement the simultaneous operation of tubes 20 and 2|during a brief period after switch I! is-shifted will produceoscillations, due to the coupling between the circuits of .the two tubesthrough the coils of transformers 29 b and 32, which will be transmittedover line It! and heard in the receiver of the handset at the rate atwhich the. charge leaks oil of the condenser. With this arrangement,since both tubes will be functioning simultaneously during said period,the coupling between the tube circuits will generate audibleoscillations that will be transmitted over the line It to the receiverat the 0pposite end of the line, informing the operator at that end thatthe other set has been switched from transmitting to receiving position.

while two embodiments of the invention have been described, it will beunderstood that other embodiments may be employed without departing fromthe invention as set forth in the claims.

In particular, the tubes may be placed in and out of operation byvarious known changes in their circuits and inparticular by changing thepotentialsapplied to various electrodes, such as '-additional controlgrids.

What is claimed is:

1. A telephone transmitting and receiving unit comprising a transmittingsystem including an amplifying vacuum tube, a receiving system includingan amplifying vacuum tube, switch means for placing said systemsalternately in operation and means for automatically generating aswitchover signal comprising means for maintaining the transmittersystem tube in operation during a signal period following actuation ofthe switch means to place the receiving system in operation. andcoupling means connecting said systems for generating signaloscillations in the transmitter system during said period.

2. A transmitting and receiving unit as set forth in claim 1, in whichthe coupling means includes a line transformer coupled to thetransmitting system tube output and to the receiving system tube inputand a second transformer coupled to the receiving system tube output andto the transmitting system input, and in which said unit includes amicrophone circuit in the transmitting system and a receiver circuit inthe receiving system, both of 'said circuits being opposite end of theline. This arrangement has the advantage of battery economy since onlythe filament of the tube that is in use is heated.

Another embodiment is illustrated in Fig. 2, and employs a similararrangement ,of transmitterand receiver circuits, input and outputtransformers and double-pole switch, which are indicated by the samenumerals as similar parts in Fig. 1. In this arrangement, however, thefilaments. 22,- 36 are continuously heated by batteries 24 grounded atthe negative pole in the usual way. Controlgrids 21 and ll! aremaintained at a negative voltage sufficient to block grounded battery66. Said potential has a value appropriate to overcome the blocking biason the control grid and to place each tube in operation when its screengrid is connected to battery 88 by switch it. v A condenser 81 isconnected between line 65 and ground, and is of appropriate capacity tomaintain on screen grid ll of the transmitter tube 2| a potentialsufficient to keep said tube in operation for a brief signal periodaftertube 20 has been placed in operation by throwing switch it fromtransmitting to receiving position, the duration of the signal periodbeing determined by the coupled to the second thansformer.

3. A telephone transmitting and receiving unit comprising a transmittingsystem including an amplifying vacuum tube, a receiving system includingan amplifying vacuum tube, and means for placing said systemsalternately in operation and automatically generating a switch-oversignal, comprising switch means for alternately energizing the'fllaments of said tubes, and coupling means connecting'said systems forgenerating signal oscillations in the transmitter system throughout theperiod of simultaneous operation of said tubes and systems during theper. sistence of useful emission in the deenergized tube.

4. A telephone transmitting and receiving unit comprising a transmittingsystem including an including means for alternately applying blockingand unblocking potentialsto' electrodes of said tubes, and means forautomatically generating a switch-over signal comprising means formaintaining said unblocking potential on the transmitting tube electrodeduring a signal period following the application of said unblockingpotential to the receiving tube electrode, thereby maintaining bothtubes in operation during said signal period, and coupling meansconnecting said systems for generating signal oscillations in thetransmitting system during said period.

5. A transmitting and receiving unit as set forth in claim 4, in whichsaid tubes are each provided with a screen grid and a control grid, bothcontrol grids being maintained at blocking potentials, and in which thescreen arids are a1: ternatelyconnected to asource of unblockingpotential by said switch means. 7

1 6. A transmitting and receiving unit as set v forth in claim 4, inwhich the means for main-- taining unblocking potential on thetransmitting tube during a signal period includes capacitance

